Multispectral Imaging 

Spectral Imaging with Filters

The ultimate goal of astronomical imaging is to obtain information on size and shape of distant objects and details of their colors or spectra. Traditionally, this information has been acquired sequentially using cameras and then spectrographs. IFIRS is an imaging spectrograph which combines the functions of a camera and a spectrograph and yields a spectrum for every pixel in the field of view.

A common example of a multispectral imager is color TV. A color TV splits the incoming light into red, green, and blue channels. In a color TV dichroic beamsplitters split the light into its component colors which are then directed a focal plane array (FPA) for detection.

An ideal astronomical imager would split the incoming light into multiple spectral channels. A typical astronomical observation might consist of standard photometric band-passes such at those shown here:
Note that real filters are not 100% efficient - filters suffer reflection losses and light is absorbed in the filter substrate and coatings. For three channels the color TV approach might be practical, but frequently many more than three channels are needed. For example, the Hubble Deep Field was observed sequentially with a single FPA and four filters centered at 300 nm, 450 nm, 606 nm, 814 nm. Spectra used to measure redshifts typically have hundreds or thousands of channels, in which case it becomes impractical, inefficient, and inflexible to use hundreds of dichroics and FPAs. In contrast, an imaging Fourier transform spectrometer is practical, efficient, and flexible.

Spectral Multiplexing

The sequential use of filters, e.g, Hubble Deep Field, wastes light. Only the radiation falling within the filter band-pass is transmitted to the FPA. The remaining light is either absorbed in the filter or reflected. The IFTS escapes this waste by detecting all wavelengths simultaneously. An IFTS is said to be spectrally multiplexed. To see how spectral multiplexing works consider an example where three bands are observed.